This invention relates to particulate additives for polymers comprising finely-divided particles with a surface coating to improve dispersibility and/or serve as a protective coating.
A wide variety of finely-divided particulate materials are known for use as additives for incorporation into polymers. These finely-divided particulate materials include, for example, TiO.sub.2 used as a white pigment or delustrant, colored pigments, and inorganic particles which can impart electrical conductivity. Many of these finely-divided particulates are difficult to disperse into polymers and poor polymer quality may result from inadequate dispersion.
Some types of finely-divided particulate additives have inorganic surface coatings to impart specialized characteristics to the additives. A problem encountered in the use of such finely-divided particulate additives is the leaching of materials from the inorganic surface coatings which can cause problems during procedures for dispersing the particles into the polymer. In addition, some materials which leach from the particles can cause problems in subsequent melt processing operations, particularly when the polymer is used for fiber manufacture. For example, one type of TiO.sub.2 especially suitable for use as a delustrant in fibers has coatings of SiO.sub.2, Al.sub.2 O.sub.3, manganese, and phosphate which avoid a direct contact between the polymer and the active TiO.sub.2 surface. Furthermore, the manganese coating suppresses undesirable photocatalytic reactions in the polymer providing the yarns with better lightfastness and stability. However, during preparation of an aqueous additive slurry for incorporation into polymer such as polyamides and polyesters prepared from aqueous solutions, it has been discovered that the inorganic elements in the coatings, especially manganese, leach from the additive into the water thereby decreasing light stabilization. In addition, the presence of these elements in the polymer causes deposits in the polymerization reactor and such deposits cause problems in the manufacture of fibers such as increased filament breaks, excessive pack pressures, spinneret plugging, etc.
Another type of TiO.sub.2 known to cause problems in fiber manufacture has a coating of SnO.sub.2, Sb.sub.2 O.sub.3, and SiO.sub.2 and is used to impart electrical conductivity (antistatic properties) to sheath/core nylon fiber. It has been learned that this type of TiO.sub.2 holds a high level of moisture and, when the TiO.sub.2 is added to a polymer such as nylon by melt blending, the moisture is released into the polymer thereby decreasing polymer viscosity which causes spinning difficulties.
Surface coatings of, e.g., triethanolamine, trimethanol propane, or polydimethylsiloxane have been used on finely-divided particulate additives. However, such known coatings have not been as effective in providing the desired degree of dispersibility for many applications. Coatings of trifunctional compounds such as triethanolamine are believed to be undesirable for use in polymers such as polyamides since they can cause branching thereby degrading the polymer. Moreover, known coatings have not provided an adequate protective coating, particularly for particles with one or more inorganic surface coatings.